Generally speaking, this application discloses techniques of measuring concentration information for one or more gasses in a fluid environment with gas sensors.
Gas sensors may be sensitive to a partial pressure or concentration of one or more gasses in a fluid environment. Some types of gas sensors may include those described in U.S. Pat. No. 5,279,795 or U.S. Patent Publication No. 2010/033214. Such sensors may be sensitive to one or more types of gas such as H2 or O2. Gas sensors may be designed to provide measurements from which a gas pressure can be estimated. Such measurements may correspond to variations in resistance and/or capacitance of a gas sensor in response to changing concentrations of gasses. A gas sensor may include palladium, such as a palladium-nickel alloy or a palladium metal-oxide semiconductor. Some mechanisms of resistance and capacitance variations in gas sensors are explained in Bridging the Pressure Gap for Palladium Metal-Insulator-Semiconductor Hydrogen Sensors in Oxygen Containing Environments, M. Johansson et al., Journal of Applied Physics, Vol. 84, July 1998 and R. C. Hughes et al., Solid-State Hydrogen Sensors Using Palladium-Nickel Alloys: Effect of Alloy Composition on Sensor Response, J. Electrochem. Soc., Vol. 142, No. 1, January 1995. It should be emphasized that the techniques disclosed in this application are in no way limited to such gas sensor structures or physical mechanisms.
Some gas sensors, such one that has a PdNi lattice, may have increased resistance or capacitance as concentration information for H2 increases. For example, resistance may increase linearly in proportion to the levels of hydrogen in a PdNi lattice, which in turn is related to gaseous H2 pressure as described by Sievert's law. This law holds that, at moderate pressures, concentration for hydrogen dissolved in solid metals is approximated by the following relationship:c≈s*p1/2 where c is the concentration of dissolved hydrogen in equilibrium with gaseous hydrogen at pressure p, and s is Sievert's parameter.
For a given application, a gas sensing system can be designed to detect the pressure of a target gas, for example, H2. In addition to the target gas pressure, however, there may be factors that can influence a gas sensor's measurements. For example, temperature of the gas sensor may influence measurements. To address this issue, a heater may be used to maintain the gas sensor within a desired temperature range. In addition to temperature, other factors may influence gas sensor measurements, such as a bias voltage applied to the gas sensor or the overall pressure of the fluid environment.
As another example, non-target gasses, such as O2, may influence a gas sensor's measurements. The presence of non-target gasses may influence or interfere with target gas measurements in at least two ways. First, because the sensor is responding to both a target and a non-target gas, the sensor reading may be too high or too low. In this respect, the influence of the non-target gas may be thought of as an offset to the target gas reading. Second, the presence of a non-target gas can alter the way a sensor measures a target gas. For example, non-target gasses can occupy receptor sites inside or on the surface of the lattice. This leaves less available receptor sites, thereby making the sensor less sensitive to the target-gas. As another example, in a PdNi gas sensor, the presence of oxygen in the lattice may affect the resistive or capacitive characteristics of the sensor. Thus, an oxygen-permeated lattice may respond to the presence of hydrogen in a different way than if the lattice was not permeated with oxygen. When oxygen permeates the lattice adsorption of hydrogen results in the formation of molecules such as H2O, OH, etc. These molecules may, by themselves, influence the resistive or capacitive characteristics of the gas sensor.
One attempted solution for reducing the influence of non-target gasses may be to use a blocking coating on a gas sensor to filter such non-target gasses. However, such a filter may reduce a gas sensor's sensitivity or response time. Another attempted solution may be to use multiple gas sensors to specifically detect non-target gasses to determine and account for concentration information for non-target gasses. However, such a solution may be expensive and/or introduce additional system complexity. Yet another attempted solution is to simply limit gas sensing applications to ones that do not include interfering gasses.
As another example of gas sensor influences, the character of a gas sensor tends to change over time, thus causing “drift.” One solution to such a problem is to repeatedly recalibrate a drifting sensor manually.
In view of the foregoing, it may be useful to provide a gas sensing system that reduces the effects of interferences with the gas sensor measurements of a target gas.